1. Field of Use
This invention generally relates to the filtration of liquids containing suspended or dissolved solids.
2. Prior Art
Filtration of liquids containing dissolved or suspended particles involves forcing the liquid under pressure against a barrier permeable to the liquid but not to most of the solids. In dead-end filtration the filter medium gets saturated with the solids and is replaced periodically. In cross flow filtration the liquid flows under pressure along the membrane filter surface at a high velocity to prevent the membrane surface from being completely covered by separated solids. This feature allows the membrane to be used for a longer period.
Cross flow membrane filtration spectrum involves filtering of particles about 10 microns (0.010 mm) to single molecules about 10 Angstoms (0.001 microns) in size. This range is divided into four classes called microfiltration, ultrafiltration, nanofiltration and reverse osmosis in the decreasing order of particle size. As the size of the particles to be filtered decreases the pressure required to filter increases.
The major challenge in industrial membrane filtration is the design of membrane modules that can pack large membrane areas into small volumes and that can withstand high pressures required in filtration.
In U.S. Pat. No. 3,367,504 a spiral membrane module is taught which consists of a central pipe for carrying the filtered liquid called permeate, and a membrane envelope and a grid material called feed spacer. The membrane envelope comprises of two permeable membrane layers sandwiching a third layer called permeate spacer which is sufficiently strong to withstand the pressure required in filtration and sufficiently porous to allow the flow of the permeate.
The membrane envelope is sealed along three ends and the fourth end is connected to the central tube so that the permeate can enter the central tube through perforations in the central tube. The feed spacer grid is placed on the membrane envelope and the two are wrapped around the central tube in the form of a spiral.
The spirally wrapped membrane module is placed inside a cylindrical vessel which provides hydraulic separation of feed and permeate streams. The feed liquid is pumped axially along the feed spacer grid from one end of the module to the other end. The filtered liquid enters into the permeate spacer in the membrane envelope, moves along the spiral path to the central tube, enters it through the perforations, and leaves at the end of the tube.
Spiral membranes consisting of several membrane envelopes, and feed spacer grids are also constructed to shorten the permeate flow path which reduces the pressure drop in the permeate side.
The spirally wound membrane allows a large membrane area to be packed into a small volume. It can also withstand high pressures. The major disadvantage of the spiral configuration is its inability to accommodate suspended particulate matter due to fouling of the feed spacer grid. It is desirable to have a spiral membrane module that has a high packing density, can withstand high pressures, and also accommodate feed liquids containing particulate matter.